1. Field of the Invention
The present invention relates to an upconverter using a two port parametric amplifier, in particular for use in magnetic resonance imaging (MRI) systems.
2. Description of the Prior Art and Related Subject Matter
MRI scanners use a combination of a strong constant magnetic field (B0) from a superconducting magnet which is modified by gradient fields generated by gradient coils, together with a rotating magnetic field (B1) from a radio frequency (RF) antenna to excite nuclear magnetic resonances in the body that generate short term RF signals that are received to build up a tomographic image.
All current-generation MRI scanners employ arrays of local coils mounted in close proximity to the scanned patient to receive the RF with maximum possible signal to noise ratio (SNR). The local coils that receive signals from the back of the patient are mounted in the patient table. Local coils that receive signals from the front of the patient are arranged into ‘mats’ that are carefully placed over the patient. Associated with each mat is a flexible cable typically containing one co-axial line for each local coil. The cables interact with the B1 field and with the signals generated from the patient so ‘traps’ (high impedance sections) must be included at regular (typically λ/8) intervals. These add cost and inconvenience to the structure. In use, the requirement to connect the cables and sterilise them between scanning one patient and the next leads to down-time between scans. Elimination of these cables is therefore desirable.
In co-pending patent application no. 0903722.7, corresponding to U.S. Ser. No. 12/612,831 filed Nov. 5, 2009, a wireless MRI system is described in which an array of antennas lining the MRI scanner bore is arranged to transmit a local oscillator (LO) signal that is received by antennas on the patient mat and fed therefrom to upconverters which upconvert magnetic resonance signals received from local coils in the patient mat and re-radiate them from the same antenna to be received at one or more of the bore array antennas for downconversion and processing to generate an MRI image.
However, wireless implementations give rise to new problems. When using a two port parametric amplifier in a wireless implementation, it is desirable that the parametric amplifier is entirely powered by an incident local oscillator (LO) signal at a microwave frequency. When there is no incident LO signal, there is no DC bias voltage, so the capacitance of the varactor diodes in the parametric amplifier is high. This leads to the parametric amplifier output being tuned to a lower frequency than its nominal tuned frequency. The resulting low impedance presented to the local oscillator loads the signal and reduces the available LO voltage. Where the LO voltage is used to generate a bias voltage, then can result in the parametric amplifier being locked in a stable, non-functional, state. Although this can be dealt with by using a self start circuit, such circuits add complexity.